A sintered R-T-B based magnet is known as a permanent magnet with the highest performance. R is at least one of rare earth elements, and necessarily contains Nd and/or Pr. T is at least one of transition metals, and necessarily contains Fe. A sintered R-T-B based magnet is used for various uses including various motors such as a voice coil motor (VCM) of a hard disc drive, a motor for an electric vehicle (encompassing EV, HV and PHV), a motor for an industrial device and the like, and also electric and electronic home appliances.
A sintered R-T-B based magnet includes a main phase formed of a compound having an R2T14B-type crystal structure and a grain boundary phase located at a grain boundary of the main phase. The R2T14B phase as the main phase is a ferromagnetic phase and mainly contributes to a magnetization function of the sintered R-T-B based magnet.
R contained in the R2T14B phase, which is a main phase of the sintered R-T-B based magnet, contains a light rare earth element RL (mainly Nd and/or Pt). It is known that replacement of a part of the light rare earth element RL with a heavy rare earth element RH (mainly Dy and/or Tb) improves the coercivity HcJ (hereinafter, may be referred to simply as “HcJ”). Namely, the heavy rare earth element RH needs to be used in a large amount in order to improve HcJ.
However, the replacement of the light rare earth element RL in the R2T14B phase in the sintered R-T-B based magnet with the heavy rare earth element RH, although improving HcJ, decreases the remanence Br (hereinafter, may be referred to simply as “Br”). Therefore, it is demanded to improve HcJ with use of a minimum possible amount of the heavy rare earth element RH so that Br is not decreased. The use of the heavy rare earth element RH is demanded to be decreased also because the heavy rare earth element RH is a rare metal.
According to a recently proposed method for improving HcJ of the sintered R-T-B based magnet, a heavy rare earth element RH such as Dy, Tb or the like is supplied to a surface of the sintered R-T-B based magnet and diffused to the inside of the magnet. Thus, HcJ is improved while the decrease in Br is suppressed.
According to a method described in Patent Document No. 1, a sintered body and a bulk body containing a heavy rare earth element RH are spaced apart from each other while a net or the like formed of Nb is present therebetween, and the sintered body and the bulk body are heated to a predetermined temperature. In this manner, the heavy rare earth element RH is supplied from the bulk body to a surface of the sintered body while being diffused to the inside of the sintered body.
According to a method described in Patent Document No. 2, powder containing at least one of Dy and Tb is put on a surface of a sintered body and heated to a temperature lower than the sintering temperature, so that at least one of Dy and Tb is diffused into the sintered body from the powder.
According to a method described in Patent Document No. 3, a plurality of sintered R-T-B based magnets and a plurality of RH diffusion sources containing a heavy rare earth element RH are loaded into a processing chamber such that the sintered R-T-B based magnets and the RH diffusion sources are movable with respect to each other and contactable with each other, and are heated in the processing chamber while being moved continuously or intermittently. In this manner, the heavy rare earth element RH is supplied from the RH diffusion sources to a surface of the sintered R-T-B based magnets while being diffused to the inside of the sintered magnets.